This invention relates to semiconductor wafer processing, but more specifically, to a method and an apparatus to detect and disperse agglomerates prior to chemical mechanical planarization (“CMP”) polishing.
Present day wafer planarization is typically accomplished using a CMP slurry of nanometer-sized particles to polish the surface of a wafer before applying circuit patterns. A CMP slurry is 99.99% comprised of nanoparticles having a mean diameter less than 50 nanometers (“nm”). Unwanted agglomerates in the slurry typically exceed 300 nm. A typical slurry composition varies between 2% to 15% by weight (copper CMP slurries are less dense than oxide CMP slurries).
The wafer polishing process is generally carried out for each layer of a multiplayer semiconductor device. Scratching or other damage occurs to the wafer during polishing when slurry particles or agglomerate exceed a given size. Thus, detecting large agglomerates or non-uniform particles in the CMP slurry will make polishing more error-free and efficient, which increase production yield.
Thus, it is an objective of the present invention to detect and/or disperse agglomerates in a holding tank or situ before supplying the slurry to polishing pads of a CMP polisher by employing acoustic microcavitation both to detect unwanted particles or agglomerates and to disperse or breakup such particles or agglomerates. An apparatus to generate such acoustic microcavitation fields is shown, for example, in commonly-owned U.S. Pat. No. 6,395,096 entitled Single Transducer ACIM Method and Apparatus, incorporated herein.
Current CMP slurry monitoring systems include light scattering methods which generally cannot be employed in opaque media or to detect particles smaller than 60 nm. A PSS (Particle Sizing Systems) instrument, for example, relies on light scattering signatures from particles and laboriously requires that a slurry sample be highly diluted and flowing through a small capillary prior to measurement. A departure from light scattering or other methods used by instruments commercially available from Colloidal Dynamics, Inc. and Matec, Inc. Such devices are based on electroacoustic effect in response to high frequency (in excess of 100 MHz) electromagnetic waves. Oscillations are excited in colloidal dispersions, which collectivity emit sound that is detected and size-inferred.
No prior system precisely detects single particles as distinguished from the sizing of a collectivity of particles. Further, no prior system can truly monitor slurries in-line at the point of use, as they are invariably able to process only diluted samples, off-line.
The present invention, on the other hand, may provide real-time, in-line, in-liquid particle detection, counting, and characterization. This contrasts with X-ray diffraction (requiring special sample preparation) or SEM analysis (which does not work in water). The present invention requires no optical transparency, is not limited to small sample volumes, and may identify particles/agglomerates selected for size from a background of other particulates. The present invention enables real-time, in situ preclusion of even a single large agglomerate (>300 nm) from a CMP polishing pad, and may comminute agglomerates in a 70 milliliters or so reservoir or slurry stream just before being fed to the polishing pad.